1. Field of the Invention
The present invention relates to a dispersion compensating method and a dispersion compensating apparatus, and, to a dispersion compensating method and a dispersion compensating apparatus for compensating wavelength dispersion occurring in a transmission path fiber of a wavelength division multiplexer transmission system.
2. Description of the Prior art
Recently, a modulation rate per one wavelength of a wavelength division multiplexer (WDM) transmission system changes from 2.4 Gbps to 10 Gbps. In the situation, a WDM dispersion compensating method is increasing in its importance.
In the WDM transmission system, there are a method of carrying out dispersion compensation for each wavelength and a method of carrying out dispersion compensation for all the wavelengths in a lump. In the dispersion compensating method per each wavelength, the costs are very higher than the lump dispersion compensating method. On the other hand, a transmission path dispersion slope varies for each particular transmission fiber, and thus transmission path dispersion cannot be compensated completely throughout all the wavelengths in the lump dispersion compensating method. Therefore, a method exists in which the band is divided into several bands, and dispersion compensation is carried out.
In a conventional WDM transmission system, in order to lower the costs, many cases the lump dispersion compensating method is applied. In any method, a dispersion compensator is such that a dispersion compensating fiber (DCF) is cut into lengths according to dispersion amounts, is disposed according to the transmission path dispersion amount of each repeating section, and carries out the compensation.
FIG. 1 shows a configuration of one example of a dispersion compensating system in the WDM transmission system. In FIG. 1, a transmission apparatus 10 carries out wavelength multiplexing on wavelengths λ1 through λn and transmits the same to a transmission path fiber 11. A repeating apparatus 12 amplifies the wavelength multiplexed signal received from the transmission path fiber 11, supplies the same to a dispersion compensating fiber 13, again amplifies the same, carried out dispersion compensation, and transmits it to a transmission path fiber 14.
In the same manner, a repeating apparatus 15 carries out dispersion compensation on the wavelength multiplexed signal received from the transmission path fiber 14, and transmits the same to a transmission path fiber 16. In the same manner, the wavelength multiplexed signal undergoes dispersion compensation every repeating apparatus. Thus, the wavelength multiplexed signal is supplied to a receiving apparatus 18 from a transmission path fiber 17. The receiving apparatus 18 carries out dispersion compensation in the same manner as that in the repeating apparatus 12 on the wavelength multiplexed signal received from the transmission path fiber 14. After that, it separates the wavelength multiplexed signal every wavelength, and outputs the respective wavelengths λ1 through λn.
Further, for example, Japanese Laid-open Patent Application No. 2001-223640 discloses that, the number of light wavelengths to multiplex and optical repeater output are detected, and, a dispersion compensating amount in a wavelength dispersion compensator provided in a transmission side or a receiving side of an optical fiber transmission path is changed according to increase/decrease of the number of wavelengths to multiplex, a change in the light signal output power in the optical fiver due to the change of optical repeater output, or such.
In the conventional method, based on a previously measured transmission path dispersion amount, a dispersion compensating fiber of a corresponding dispersion amount should be prepared. Therefore, upon system establishment, merely wire materials can be purchased before the transmission path dispersion amount is obtained. The dispersion compensating amount product cannot be produced until then.
Further, for when a dispersion compensating menu having respective dispersion compensating fibers correspond to different dispersion amounts, and the dispersion compensation fiber product is obtained according to the desired dispersion amount, the diversion compensating menu in a very large size is required since a difference between the minimum distance and the maximum distance increases along with an increase of the length of the recent repeating transmission distance. Especially, when the lump dispersion compensating method is applied as mentioned above, the steps in the dispersion compensating menu become finer and enormous in the number, in order to compensate a permissible range of a residual dispersion of all the wavelengths.
On the other hand, along with an increase in the number of wavelengths in the WDM transmission system, a light level difference in a total level state from one wavelength through full wavelengths increases, and also, the WDM transmission system having an increased transmission span light loss is demanded.
For such a case, since an optical amplifier is applied to amplify a lump of wide band wavelengths in the WDM transmission system, the optical amplifier's ASE (amplified spontaneous emission) light becomes larger than the signal power. Therefore, it is necessary to carry out ASE correction to correct the ASE light, and thus, adjust the output power of a signal light component to the optical fiber to a designed value. This is because, as the output power increases, the dispersion compensating amount changes, and also, error may occur due to nonlinear effect.
Further, the maximum permissible transmission path loss of the current WDM transmission system is limited by the minimum wavelength number (upon one wavelength usage) interruption detection/restoration level. That is, a distance within which a subsequent stage optical amplifier can recognize light input and start up corresponds to the maximum permissible transmission path loss.
There, when the wavelength number is small, nonlinear effect called four light wave mixture (FWM) can be inhibited by an increase of the wavelength interval. Therefore, an increase of the output power upon a small wavelength number such as one wavelength may result in an increase of the permissible maximum transmission path loss (an increase in the distance within which the subsequent stage optical amplifier can recognize the light input and start up). However, the output power increase results in a change in the dispersion compensating amount, and also, results in an influence of self phase modulation (SPM) becoming remarkable.
On the other hand, development of a variable dispersion compensator is proceeded with as a device in which the lump dispersion compensation can be made and its dispersion amount can be changed. However, the variable dispersion compensator is such that, when the dispersion compensating amount is changed, a passing wavelength band changes. As a result, ALC control of an optical amplifier provided together with the variable dispersion compensator may malfunction, and thus, OSNR (optical SN ratio) may degrade.